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Naval Meteorology and Oceanography Professional Development Center. Fundamentals of Weather. Contents Air Masses Fronts (cold, warm, occluded, stationary) Pressure Systems Clouds Winds. cA. cP. mP. mP. cT. mT. mT. Air Masses.

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naval meteorology and oceanography professional development center
Naval Meteorology and Oceanography Professional Development Center

Fundamentals of Weather

  • Contents
  • Air Masses
  • Fronts (cold, warm, occluded, stationary)
  • Pressure Systems
  • Clouds
  • Winds
air masses

cA

cP

mP

mP

cT

mT

mT

Air Masses

Air masses are large bodies of air that have similar horizontal temperature and moisture characteristics.

Air masses are classified by

where they originate (their

“source region”).

First letter:

c - continental

m - maritime

Second letter:

P - polar

T - tropical

Air masses move from their source regions.

As they do, distinct boundaries form between them.

fronts

Cool

L

Cold

Warm

B

L

Cold

air mass

(e.g, cP)

Cool

air mass

A

Warm

air mass

(e.g, mT)

Fronts

A “front” is defined as the boundary between two air masses.

Air mass characteristics on either side of a frontal boundary can be very different (point “A”) or more similar (point “B”).

Frontal intensity is defined by the the temperature and moisture differences on either side.

The stronger the differences on either side of the front, the more potential for severe weather.

fronts4

Occluded

  • Cold
  • Warm
  • Occluded
  • Stationary

L

Warm

Cold

Stationary

Fronts

There are four types of fronts.

cold front

Cool

Cold

Front

Warm

Cold

Cold Front

A cold front defines the boundary between an advancing cold air mass and a warm air mass.

  • Cold fronts are associated with:
  • Sharp temperature changes over relatively short distances
  • Changes in air moisture content
  • (moist before, dry after)
  • Reduced visibility in showers
  • Wind shifts with frontal
  • passage
  • Pressure changes with
  • frontal passage
  • Unstable cumuliform
  • clouds and showery
  • precipitation patterns
cold front6

Slow-moving cold front

cold

Fast-moving cold front

cold

Cold Front

Cold fronts are further characterized by their speed of movement - which defines their slopes.

  • Slow moving cold fronts:
  • Most clouds and weather are at and
  • behind the advancing cold front
  • Longer periods of rain/snow, less
  • thunderstorm activity
  • Fast moving cold fronts (steeper slope):
  • Most clouds and weather are near and
  • ahead of the advancing cold front
  • Rain/show showers (sometimes heavy),
  • more thunderstorm activity
  • Thunderstorms often form ahead of front
warm front

Cool

Warm

Front

Cold

Warm

Warm Front

A warm front defines the boundary between a retreating cool air mass and an overriding warm air mass.

  • Warm fronts are associated with:
  • Extensive cloud activity ahead of the front.
  • Temperature rises with frontal passage.
  • Wind shifts with frontal passage.
  • Poor visibility at and ahead
  • of the frontal boundary.
  • Thick, stratiform clouds
  • and steady precipitation
  • patterns.
  • Overall improvement
  • in weather conditions
  • with frontal passage.
warm front8

Warm front

cool

warm

Fog

Warm Front

Warm fronts have extremely shallow slopes.

  • Clouds and weather are at and ahead the advancing warm front.
  • Precipitation consists of steady rain or snow and usually no thunderstorm activity - although thunderstorms may be embedded within the frontal area and hard to discern on satellite pictures.
  • Fog is frequently found in the cooler air ahead of the warm front.
occluded front

“Triple point”

Occluded Front

An occluded front defines the portion of frontal area where the cold front has overtaken the warm front and pushed it aloft.

  • Occluded fronts are associated with:
  • Both warm front and cold front
  • weather characteristics
  • The worst weather with
  • an occluded front is
  • located where the cold
  • and warm fronts meet
  • at the surface: the triple point.
occluded front10

Warm occlusion

warm

cool

cold

Cold occlusion

warm

cold

cool

Occluded Front

There are two types of occluded fronts: warm, and cold.

  • Warm occlusions:
  • Milder maritime polar (mP) air overtakes colder continental polar (cP) air.
  • Warm occlusion weather is similar to that of a warm front.
  • More steady, less showery precipitation.
  • Cold occlusions:
  • Colder cP air overtaking milder mP air.
  • Cold occlusion weather resembles warm
  • frontal weather before the front passage,
  • and cold frontal weather during and after
  • passage.
stationary front

Cool

Warm

North

Stationary Front

A stationary front has essentially no movement (the advancing cold front has “stalled out”).

  • Stationary fronts are associated with:
  • East-west orientation.
  • Normally clear to partly
  • cloudy skies.
  • Normally little or no
  • precipitation.
stationary front12

Stationary Front

Overrunning

Warm

moist

Stationary Front
  • Stationary fronts:
  • Normally have “good” weather associated with them.
  • Exceptions:
  • If a new pulse of cold air moves in from the north, the cold front can begin to advance and a new low can form on the frontal boundary.
  • If warm, moist air overruns the frontal boundary, widespread cloudiness and light precipitation can cover a vast area.
pressure systems

H

L

Pressure Systems

There are two types of pressure systems:HighsandLows

A “high,”or anticyclone, is an area of high pressure around which the winds blow clockwise in the northern hemisphere (counterclockwise in the southern hemisphere.) High pressure is associated with sinking, more dense air.

A “low,”or cyclone, is an area of low pressure around which the winds blow counterclockwise in the northern hemisphere (clockwise in the southern hemisphere.) Low pressure is associated with rising, less dense air.

pressure measurements
Pressure Measurements
  • The amount of force exerted by air molecules over a given area of the earth’s surface is called atmospheric pressure (or “air pressure.”)
  • When the density of the air increases, pressure goes up. When density decreases, pressure goes down.
  • Barometers are used to measure pressure in different units:
    • Hectopascals (hPa) - Measured to the 1/10 (ex. 1018.8 hPa)
    • (1 Hectopascal = 1 millibar)
    • Inches of mercury - measured to the 1/100 (ex. 29.92 ins)
  • The most common type of barometer is called an “aneroid barometer.”
pressure systems15
Pressure Systems
  • High pressure is normally associated with “good” weather:
  • Clear or clearing skies, no precipitation, light winds (away from terrain effects).
  • Low pressure is normally associated with “unsettled” weather:
    • Cloudy skies, precipitation, gusty winds.
  • Barometers are used to measure pressure in different units:
    • Hectopascals (hPa) - Measured to the 1/10 (ex. 1018.8 hPa)
    • (1 Hectopascal = 1 millibar)
    • Inches of mercury - measured to the 1/100 (ex. 29.92 ins)
pressure systems isobars

20

12

16

08

1008 hPa

04

00

24

96

L

996 hPa

H

994

1025

Lines of equal pressure are called “isobars.”

Pressure Systems: Isobars

Isobars are usually drawn in 4 hPa increments.

  • Denoted by a solid black line, labeled as shown.
  • The highest and lowest pressure values within highs and lows are are depicted next to the “H” or “L” label.
pressure systems and fronts

A low pressure area forms where the cold and warm front meet.

(Cool)

L

H

High pressure defined by the air mass “moving in”

(Cold)

(Warm)

L

H

Cool

air mass

As the system develops, the position of the low moves away from the cold and warm fronts.

Cold

air mass

(e.g, cP)

Warm

air mass

(e.g, mT)

Pressure Systems and Fronts

Pressure systems and fronts have a direct relationship

(time)

pressure systems18

L

This low “fills” (dissipates) over time

L

L

L

(time)

H

New

Low

Original

Low

(time)

L

H

The “original low” fills and a new system moves off to begin the cycle again.

Pressure Systems

New Lows frequently form at the “triple point.”

slide19

5 kts

10

20

50

65

100

Wind

Wind is air in motion relative to the earth’s surface. In meteorology, wind is the observed effect of horizontal transport of air masses over the Earth’s surface. It is caused by temperature differential between 2 areas.

Wind speeds are plotted on meteorological charts as follows:

Flag = 50 kts

Long line = 10 kts (8-12 kts)

Half line = 5 kts (3-7 kts)

wind speed scales beaufort scale
Wind Speed Scales - Beaufort Scale

<

The Beaufort Scale defines seas (and sea state) according to wind speeds

wind speed scales
Wind Speed Scales
  • Wind speeds not associated with tropical systems (World Meteorological Organization).
    • 4 - 27 kts = breeze (light, gentle, moderate, fresh, strong).
    • 28 - 33 kts = near gale.
    • 34 - 47 kts = gale (gale, strong).
    • 48 - 63 kts = storm (storm, violent).
    • 64 kts and greater = hurricane force.
  • Wind speeds associated with tropical systems.
    • Less than 34 kts = Tropical Depression.
    • 34 - 63 kts = Tropical Storm.
    • 64 - 129 kts = Hurricane.
    • Greater than 130 kts = Super Hurricane.
pressure systems isobars22

Wind barbs define wind direction and speed on a synoptic chart...

20

12

16

08

04

00

24

96

L

H

994

1025

…and also help define frontal boundaries

Pressure Systems: Isobars
clouds
Clouds
  • Clouds are:
  • Water molecules suspended in the atmosphere.
  • Three things are required for cloud formation:
  • - Moisture
  • - Cooling
  • - Condensation nuclei (something for
  • the moisture to condense on)
  • There are three general types of clouds:
  • Cumuliform
  • Stratiform
  • Cirriform
cumuliform clouds
Cumuliform Clouds

Cumuliform clouds are unstable, vertically developed, and have generally distinct edges. They are formed either by convective action (daytime heating) or mechanical lifting (cold front). Showeryprecipitation is associated with cumuliform clouds. Cumulonimbus clouds are clouds with extreme vertical extent and are associated with heavy precipitation and thunderstorms.

Stratiform clouds are stable and form indistinct layers. Steady, light precipitation is associated with stratiform clouds. A particular form of stratiform clouds, nimbostratus, is associated with heavy, steady precipitation. Fog is nothing more than a form of straitiform clouds (stratus) that has reached the ground.

Cirriform clouds are located at higher altitudes and are composed completely of ice crystals.

clouds and the atmosphere
Clouds and the Atmosphere
  • For meteorological purposes, the atmosphere is divided into three levels (“etages”).
  • Low etage - Surface to 6500 ft (middle latitudes)
  • Middle etage - 6500 ft to 23,000 ft
  • High etage - 16,00 ft to 43,000 ft
  • Different cloud types are associated with the low, middle, and high etages. More common cloud types are shown below. Some “stay” in their etages, some extend through one or more (*):
  • LowMiddleHigh
  • Cumulus, Cumulonimbus * Altostratus Cirrus
  • Stratocumulus Altocumulus Cirrocumulus
  • Stratus Nimbostratus * Cirrostratus
  • (Cumulonimbus begins in the low etage and builds into the mid and high etages)
  • (Nimbostratus frequently begins as a “mid” cloud and descends into the low etage)
clouds and fronts example

Cirrostratus

Altostratus

Nimbostratus

Stratus (fog)

L

Altocumulus

Altostratus

Cirriform

Stratocumulus

Cumulus

Cumulonimbus

Altocumulus

Cirrocumulus

Cumulus

Stratocumulus

Clouds and Fronts - Example